The goal of this research is to understand the mechanisms that maintain the integrity of complex motivated behaviors under different conditions. Towards this goal, mechanisms that maintain coordination of antagonist muscles in the feeding system of Aplysia will be investigated under different motivational states. A model is presented, in which peptides are proposed to compensate for the increased frequency and intensity of contractions during highly motivated feeding, while stretch-activation and spiking exhibited by the weaker muscles are proposed to compensate for variations in the contractions of their more powerful antagonists. This model will be tested using a combination of behavioral, electrophysiological, biochemical, and biophysical techniques. Firing characteristics of motor neurons in different motivational states will be determined from chronic in vivo recordings. These characteristics will be mimicked by motor neuron stimulation in vitro to determine the quantities of peptides released, as measured by radioimmunoassay of muscle perfusates. Cellular mechanisms will be probed with biophysical techniques, and based on the results, these mechanisms will be perturbed in a reduced preparation to test the model. A conceptual model that incorporates a variety of modulatory mechanisms will be tested, and if correct, will help guide future studies on the integrative role that modulatory mechanisms play in the coordination complex behaviors. These studies should facilitate understanding of the neural basis of complex behavior and of their dysfunction as seen in prevalent mental illnesses such as depression and mania.